This invention relates to touch probes useful in conjunction with coordinate measurement machines and particularly relates to piezoelectric touch probes.
Touch probes are used by Coordinate Measurement Machines (CMM) to measure the geometry of structures. A typical multi-degree of freedom CMM consists of three hardware components:
1) a robotic arm;
2) a motorized probe head; and
3) a touch probe.
The robotic arm moves the probe head along the X, Y and Z axis. The motorized probe head rotates the touch probe in the vertical and horizontal planes. The use of a motorized probe head thus transforms a 3-axis CMM into a machine having 5 degrees of freedom. A workpiece with complex geometry including holes and cavities requires this capability. Utilizing position feedback and the touch sensitive probe, a CMM works by either tracing or interval touching a workpiece. Data collected by a CMM are used by engineers and manufacturers to document and qualify part geometry. A state-of-the-art CMM can be calibrated to a degree of accuracy of less than 0.0002 inches (0.2 mils). Highly sophisticated computer systems are utilized to automate the entire measurement process including programming the approach path of the probe""s ruby tip stylus, adjusting probe speed and data collection and analysis.
Most commercially available touch probes operate by measuring a change in position, pressure and/or force produced when a ruby-tipped stylus makes contact with a test structure. When the stylus feels the surface of the test structure, the minute force of contact between the stylus and workpiece activates a switch or pressure/force sensor to trigger the CMM. The utilization of changes in position, pressure and/or force as the triggering event thus limits the prior art touch probes to relatively low measurement speeds. If used at higher operating speeds, the prior art probes will produce a false trigger due to the seismic effect of the ruby-tipped stylus. In an effort to minimize this problem, prior art touch probes have provided a means to adjust the trigger force level. Touch probes that trigger due to stylus position also have adjustable springs to increase or decrease the contact force needed to deflect the stylus thereby causing it to trigger. If sufficient contact force is produced between the stylus and the workpiece the touch probe will trigger the CMM to stop and collect position data. In the case of rigid elastic parts, the level of contact force required to trigger a measurement is usually sufficiently low so that CMM accuracy can be maintained. However, even when the lowest trigger force settings are utilized, test structures that are flexible, fragile and/or soft will deflect or even break prior to the touch probe triggering a measurement. Although this condition does not affect the measured precision of test data, since trigger levels are repeatable, it will nevertheless generate inaccurate geometric data. For example, workpiece deflections of from 0.001 to 0.015 inches have occurred prior to the touch probe triggering. This deflection often exceeds the 0.0002 inch accuracy to which the machine is calibrated. Additionally, if used with brittle ceramic and fragile glass parts, breakage will occur due to the touch force required to trigger the CMM to stop. Under such conditions, the CMM operator can not determine the accuracy of measured part data by conducting repeatability tests since trigger levels are consistent. While the repeatability of prior art measurements is quite high, the accuracy of the data collected can often be quite low especially for flexible test structures.
Thus, if a touch probe could be provided which was (1) compatible with existing CMM systems, (2) useful for measuring both rigid and flexible parts, (3) operative at higher speeds without producing false triggers, and (4) capable of yielding results approaching the accuracy of the CMM calibration specification, a long felt need would be satisfied.
U.S. Pat. No. 5,247,751 to Ohya et al discloses a touch probe that comprises an ultrasonic vibrating horn having a piezoelectric element. This invention uses a RF (Radio Frequency) electrical signal to drive the piezoelectric element(s) which is sandwiched between electrodes and designed to vibrate at the mechanical natural frequency of the ultrasonic vibrating horn, and a feeler to be brought into contact with an object to be measured. The probe further comprises a touch detecting device for monitoring a current between said electrodes to detect a touch between the object to be measured and said feeler in accordance with a change in current value produces at the moment said feeler touches the object to be measured.
The vibrating horn consists of a stylus-like structure with a large-diameter base. The horns geometry is based on the quarter-wavelength equation for compression or longitudinal waves. By electrically driving the piezoelectric crystals at the quarter-wavelength frequency, a standing wave is produced in the stylus.
The patent to Ohya et al differs from the instant invention as follows:
1) The instant Piezoelectric Touch Probe (PTP) uses a piezoelectric driver to vibrate a high sensitivity accelerometer that is coupled to a stylus/suspension unit. Acceleration, not current, is used to detect contact conditions. Both the piezoelectric driver and accelerometer can be designed for high amplitude drive and resonant detection.
2) The stylus/suspension unit are flexible permitting significant deflection without taking on a permanent set. Stylus/suspension units are made from low modulus, high yield spring material. The stylus/suspension unit maintains the accuracy of CMM measurements by deflecting away from the part without causing bending of the part. The stylus/suspension unit is designed with a globe stiffness. This means that contact angle between a part and stylus does not significantly change interface stiffness.
3) The stylus/suspension unit vibrates using a transverse-mode and/or axial-mode of vibration at a high modal frequency (typically  greater than =5th mode of transverse/axial vibration). This technique sensitizes the entire stylus shaft. Sensitizing the entire stylus shaft reduces shaft-out problems that occur when the CMM conducts a search for points that define the surface. Errors in measured points can result in shanking of the stylus during a measurement process. Shanking occurs when the stylus shaft, not the ruby tip, makes direct contact with the part. CMM software uses all data points including shank touches to locate the calibration points.
U.S. Pat. No. 5,524,354 to Bartzke et al and entitled xe2x80x9cProbe Element for Coordinate Measurement Systemxe2x80x9d utilizes an array of piezoelectric beams arranged in a polygonal configuration. The patent focuses on the geometry of the piezo-resonator element. This patent claims piezo resonators in the form of rod oscillators, tuning-fork oscillators, torsional oscillators, longitudinal oscillators, quarter-wave oscillators and plate-shaped oscillators. The tactile sensing of the micro-probe elements on the surface of the specimen is effected by measuring the change in the resonance characteristics of the piezo-resonators.
This patent does not resonate a piezoelectric element coupled to a structure, thus the stylus is comprised of the so-called piezo-resonator, or a plurality thereof. This patent does not use a vibration sensor to detect contact. Due to this design, the touch probe system depicted in the patent will be very stiff and very brittle. The stylus would not flex when making contact with a part, thus breakage or unacceptable error would likely occur. Additionally, breakage of either the probe or the workpiece may well occur at high impact (touch) speeds.
The instantly presented PTP does not use a change in vibration level of a resonant piezoelectric element or piezo-resonator to detect contact. The PTP of the instant invention uses a dynamically tuned structure that resonates the piezoelectric driver signal and is further amplified by a resonance design for the accelerometer. This approach amplifies the sensitivity and output signal level of the PTP. A high sensitivity and output signal level for the PTP is required to detect any minute change in mechanical impedance that occurs during contact along the stylus shaft or at the end.
The instant invention is directed to a dynamic probe that is sensitive to contact and consists of five subsystems:
(1) piezoelectric vibrator controlled by a self-tuning drive circuit that adjusts operating frequency and amplitude;
(2) a high sensitivity, resonant accelerometer unit coupled to the piezoelectric vibrator;
(3) a flexible suspension/stylus with transverse resonant frequencies tuned to the operating frequency band of the piezoelectric vibrator;
(4) a triaxial set of piezoelectric load cells to measure and detect; (a) suspension/stylus vibration and (b) suspension/stylus stress/force generated at contact with the workpiece; and
(5) a digital control circuit that automatically sweeps the operating band of the piezoelectric vibrator to
(a) detect the resonant frequency of the stylus/suspension,
(b) set the drive signal frequency at the resonance of the stylus/suspension, and
(c) control the drive signal amplitude.
The digital control circuit also measures the output signal from the high sensitivity resonant accelerometer and compares this signal to a reference level to determine a touch or no touch condition and produce a trigger signal.
The high sensitivity, resonant accelerometer assembly has two preferred embodiments. In the first embodiment, a dynamically tuned structure is provided to amplify driver vibration levels and fasten the stylus/suspension. This mechanism couples the piezoelectric driver to the high sensitivity, resonant accelerometer. In the second embodiment, the piezoelectric vibrator is directly coupled to a high sensitivity, resonant accelerometer having a low seismic mass. In this embodiment, the accelerometers seismic mass supports the stylus/suspension.
The digital control unit provides a means for periodic calibration of piezoelectric driver frequency and amplitude for resonance stylus/suspension dynamics.
A low frequency triaxial set of piezoelectric load cells couples the suspension/stylus to the CMM probe head. This sensor group measures the dynamic response of the stylus in contact with the test part. This measurement detects the presence of vibration and high suspension/stylus impact forces during CMM operation.
Thus, it is an objective of the instant invention to provide a touch probe which is compatible with existing CMM systems, is useful for measuring both rigid and flexible parts at high CMM operating speeds, and yields results approaching the accuracy of the CMM calibration specification.
It is a further objective of the instant invention to produce a piezoelectric touch probe which combines a piezoelectric driver which vibrates a high sensitivity accelerometer which is coupled to a stylus/suspension unit.
It is yet a further objective of the instant invention to teach the use of an acceleration signal to detect a contact condition.
It is a still further objective of the instant invention to teach a dynamically tuned structure to amplify the driver signal.
It is yet another objective of the instant invention to provide a resonant accelerometer designed to amplify the acceleration signal.
It is still another objective of the instant invention to teach a stylus/suspension design which is coupled to the dynamically tuned structure.
It is yet an additional objective of the instant invention to teach a flexible, low modulus, high yield strength ruby-tipped stylus/suspension with a significantly low stiffness such that contact force between the stylus and the part are minimized.
These and other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.